Upgraded Rear Differential Options...
 

With Guru going out of business it seems, it makes it quite difficult to obtain their billet rear differential unit that bolts into the factory housing. I was just wondering if Guru was/is the only option for a complete bolt-in 600HP capable (need some headroom) unit available for the NAs. I know I could always swap to the T-II diff or even Ford 8.8, but I'm trying to keep custom axles and hubs to a minimum if possible. If not, I guess I don't have a choice... Thanks in advance guys for the information!

Still haven't seen a broken Miata 7" ring and pinion. Stick an OS Giken in a 7" and see what happens.

Thanks for the reply Andrew... I'm just afraid that a turbo 13B-RE will grenade the diff pretty quickly. Then again, I could be wrong...

Give Emilio at 949 Racing a call. His website describes the OSG diff as "uttery bombproof even with big boosted horsepower and Hoosiers", so your car might make a very good test of it. If it fails, he was wrong. If it survives, your car gets to go on the website as a testimonial.

"My name is Calton, and not even my 600 HP rotary-powered Miata was man enough to break the OS Giken."

I've built an OS Giken 7" rear diff. Used the S2K diff in a MSM pumpkin with MSM axles. I think it will be pretty strong.

What about the front differential?

Going with an ARB locker.

Thanks Joe. I might take this challenge. :D

[Gary Coleman]What you talkin' 'bout hustler?[/Gary Coleman]

Theres always the ford option, several ratios to choose from.
but youll loose the PPF which I think would be a bad thing

Loosing the PPF is in the plan anyways as this will be mated to a Ford T5 Tranny...

Install of the T5 will be similar to this only not in an FD: http://www.rx7club.com/showthread.ph...ht=dt-5+damian

well then you might consider that

Boss frog sells a bolt-in differential subframe for the ford 8.8, and it comes with hardware, bushing and whatnot.
They also sell the axles to go with it, unfortunately theyre pretty damn expensive...

Miata V8 Swap

http://www.bossfrog.biz/pics/Image4.jpg

http://www.bossfrog.biz/pics/Image6.jpg

Hmm. I hadn't considered the Ford option. Definitely indestructible, and more R&P options.

If you do go with the OSG, I'd strongly consider using the MSM axles as they are slightly larger than the stock ones.

FTP- what's a complete Ford diff with LSD typically go for?

FTB- I actually sent Boss Frog an email about that kit and such as far as the axle clearances with the Miata control arms and coilovers. I will be running tubular control arms built by John Brakke for E-Production cars and didn't want to spend that kind of money only to find out that the Ford 8.8 install would completely make them useless in my application. But by looking at their description it seems fine, but just wanted some verification.

I'm not sure on the going rate for LSD's for the Ford 8.8 either... And thanks for the tip on the MSM axles Joe. I will look into it for sure.

Im not real sure about the details regarding the boss frog kit. I talked with the owner of racing concepts here in jax about their ford differential options and it sounded like a good way to go as far as R&P options and LSD options. But apparently people have trouble dealing with that them, so idk if you want to bother asking them about their kit.

I would imagine that the tubular control arms would have more clearance all around than stock, but I guess you never know.


Ford racing has several LSDs availible for the 8.8

Ford Racing Performance Parts [Differentials]

and lots of choices for a R&P

Ford Racing Performance Parts [Ring ]

Thanks for the links and info on dealing with Boss Frog FTB. I like the CHEAP options for the Ford 8.8 so far though. :D

It's just the unfortunate part of gaining weight on the rear end to gain power up at the engine. Sigh...

The 8.8s you want for the Miata are the IRS versions found in the TBird/Cougar and Lincoln MVIII. TBird has a steel housing, the MVIII has an AL housing. Pick & pull prices are probably under $200, a lot of times it is cheaper to buy a non running car for a little more, pull the rearend and part out the motor and trans (MKVIIIs have an all aluminum Teksid block which you can sell for $150-200 bare) and then scrap the car. I believe both the Tbird and MKVIII use a carrier w/28 spline axles. 31 spline axle carriers (often found in newer Explorers) allow you to use stronger axles, but the Factory Five IRS axles (cheapest option for an 8.8 in a Miata) are currently only available in 28 spline. TBirds and MkVIIIs typically have open carriers, so you will need to swap out the open carrier to get an LSD. Eaton, Detroit Locker, Traction-Lok, Auburn etc. all make 8.8 carriers.

The Ford 8.8 Traction Lok carrier is a clutch plate style LSD and is usually available on Stang sites (Corral.net) or Ebay for $75-$100. Traction-Loks are easy and cheap to rebuild (cheap kits on Ebay) and you can put in an additional clutch plate for more lock-up". 3.27, 3.55, 3.73 and 4.10 gears are always available used at very low prices (50-100) and new gear sets start in the $150ish range.

sn - Thanks for the reply. Seems like you have done quite a bit of research on this swap in the past. I would definitely be running the aluminum housing if I were to go the Ford 8.8 route.

Stein has done the research on Miata fitment and axles; I can't take credit for that. I know enough about Ford parts in general to be dangerous. If you are set on using the AL carrier and can't find a MKVIII unit, I think the later model Explorers (version w/the 4.6L Mod Motor V8) had an AL carrier IRS with 31 spline axles and a Traction Lok (don't know if the T-Lok was standard or optional). Unfortunately, 31 spline carriers won't work with the Factory Five axles which are the cheapest option for mating the 8.8 to a Miata. You still need to swap in Ford hubs in the Miata uprights or broach the Miata hubs for the Ford axle. Haven't personally done the swap, but it is NOT a simple drop in.

Thanks again for the reply. I am personally not dead-set on the Ford 8.8 swap as I would prefer to run something in the factory 1.8L housing to make things less complicated and lighter, but based on some of the options out there the Ford 8.8 might be a better choice down the road.

7.5" Ford
 

FWIW,

The 7.5" Ford IRS is a smaller version of the 8.8 and obviously uses a 7.5" ring gear. Earlier TBirds/Cougars (2.3L turbocharged?) used it. It is stronger the 7" Miata (I believe Martin/Montster Miata may have used this as an entry level rearend); not sure if it is stronger than the 8" TT RX7. Like the 8.8", parts are cheap and readily available. The solid axle version of the 7.5" is stock in the base 4 banger Ford Ranger and seems to hold up in V8 swaps if one avoids sticky tires and drag racing launches. Not sure what your options for axles are outside of Monster Miata.